This invention relates to an imaging medium with a low refractive index layer. More specifically, it relates to such an imaging medium in which a low refractive index layer is interposed between an image-receiving layer and a transparent layer through which an image formed on the image-receiving layer is viewed.
Multi-layered imaging media in which an image is viewed against a light scattering background are known. Such media are generally structured as a series of thin layers overlying one another and typically include a transparent image-receiving layer or layers in which the image is formed by an imagewise and depthwise distribution of image forming components. One surface of the image-receiving layer is usually in contact with a light scattering layer against which the image is viewed. In some types of imaging media, for example the integral diffusion transfer process film units described in, inter alia. U.S. Pat. Nos. 3,415,644; 3,594,165; 3,647,437; 4,367,277 and 4,740,448, the other surface of the image-receiving layer is covered with a transparent layer, which protects the rather fragile image-receiving layer during handling of the exposed film unit; this transparent layer is typically a polymeric film which serves as a support for the imaging-receiving layer. The image is viewed through the transparent layer, and is thus illuminated by ambient light, which passes through the transparent layer and the image-receiving layer, after which the light is reflected from the light scattering layer and then in part is transmitted back through the image-receiving layer and transparent layer to the viewer.
In such an imaging medium, substantial amounts of light undergo total internal reflection at the transparent layer/air boundary, since the refractive index of the transparent layer in commercial imaging media is typically around 1.64. The effects of such internal reflection in color prints have been investigated theoretically by Williams and Clapper, Journal of the Optical Society of America, 43(7), 595 (1953). This paper shows that such internal reflection accounts for staining of highlights, increase in maximum density, shortened exposure latitude, and color desaturation. From the mathematical model in the Williams and Clapper paper, one can also infer that loss of sharpness will occur when the transparent layer is of significant thickness. Similar theoretical investigations may be found in N. Ohta, Photographic Science and Engineering, 16(5), 334 (1972), which states that "[C]olor reproduction characteristics may be considerably influenced by refractive index n of binders, especially when the color prints are viewed under diffuse illuminations.", and by the same author in Journal of Applied Photographic Engineering, 2(2), 75 (1976), which states that "Color reproduction in color prints is complicated due to the non-linear relationship between reflection density and dye amount. The nonlinearity arises from surface reflection, refraction and multiple internal reflections of light flux in a gelatin layer." This paper also discusses the effect of color gamut in color prints under diffuse illumination versus refractive index of the binder. However, although all three of the aforementioned papers discuss the deleterious effects of internal reflections on the quality of a print as seen by a viewer, they do not make any suggestions for modifying the structure of the print to reduce these deleterious effects.
U.S. Pat. No. 2,481,770 describes a photographic film, of the conventional negative-producing type, with a low refractive index layer between the emulsion and the support. This low refractive index layer is stated to reduce halation by lowering the effect of total internal reflection of light at the rear face of the support or a dye backing.
U.S. Pat. Nos. 3,427,158; 3,706,557 and 4,298,674 all describe film units of the integral diffusion transfer process type, in which the image-receiving element comprises an image-receiving layer, a spacer layer, a neutralizing layer and a transparent (support) layer. An alkaline developer is released between the image-receiving layer and the photosensitive element of the film unit to develop the image. Hydroxyl ions from this alkaline developer diffuse through the image-receiving layer and the spacer layer so that, after a predetermined period, the hydroxyl ions are neutralized by the acid in the neutralizing layer and development is terminated.
U.S. Pat. No. 4,367,277 describes a film unit of the integral diffusion transfer process type, in which the image-receiving element comprises an image-receiving layer, a transparent layer and an unhardened gelatin layer disposed between the image-receiving layer and an alkaline developer. The unhardened gelatin serves as a decolorizing layer which decolorizes the part of the developer immediately adjacent the image-receiving layer, so rendering the film unit white to a viewer looking through the transparent layer during development.
U.S. Pat. No. 4,499,164 describes an image-carrying medium comprising a transparent image-receiving layer, adjacent one surface of which is disposed a layer of image dye(s) which forms the image; a light-scattering pigment layer is disposed adjacent the image dye layer. An optical barrier layer is disposed between the image dye layer and the underlying diffuse reflector, this optical barrier layer operating to minimize non-linear density effects due to multiple internal reflections within the medium. The patent states that the use of a low refractive index material in the optical barrier layer is advantageous.
It has now been found that, in an imaging medium in which a transparent layer is superposed over the image-receiving layer so that the image in the image-receiving layer is viewed through the transparent layer against a background provided by a light-reflecting layer, the deleterious effects on perceived image quality caused by internal reflection can be reduced by placing a layer of low refractive index between the image-receiving layer and the transparent layer. It has also been found that prints from certain integral diffusion transfer process film units, in which such an imaging medium is employed as the image-receiving element, display improved aging properties.